1. Field of the Invention
The present invention relates to an antenna for a mobile communication terminal, and more particularly to an antenna installed in a mobile communication terminal for processing transmitted/received signals.
2. Description of the Related Art
Recently, mobile communication terminals have been developed so as to satisfy a miniaturization and light-weight trend and provide various services. In order to meet these requirements, internal circuits and components employed in the mobile communication terminal have been developed to have multiple functions and be miniaturized. Such a tendency is also applied to an antenna, which is one of the essential components of the mobile communication terminal.
A helical antenna and a planar inverted F-type antenna (hereinafter, referred to as “PIFA”) are generally used in mobile communication terminals. The helical antenna is an external antenna fixed to the upper end of the terminal, and is used together with a monopole antenna. When an antenna assembly including the helical antenna and the monopole antenna is extended from a main body of the terminal, the antenna assembly serves as the monopole antenna, and when the antenna assembly is retracted into the main body of the terminal, the antenna assembly serves as a λ/4 helical antenna.
Such a combined structure of the helical antenna and the monopole antenna has an advantage such as a high gain. However, this combined structure of the helical antenna and the monopole antenna has a high SAR characteristic due to its non-directivity. Herein, the SAR characteristic is an index of the harmfulness of an electromagnetic wave to the human body. Since the helical antenna is protruded from the mobile communication terminal, it is difficult to aesthetically and portably design the appearance of the helical antenna. Further, the monopole antenna requires a sufficient storage space within the terminal. Therefore, the combined structure of the helical antenna and the monopole antenna limits the miniaturization of a mobile communication terminal product using this structure.
In order to solve the above problems, there has been proposed a PIFA having a low profile structure. FIG. 1 illustrates a structure of a conventional PIFA. The PIFA comprises a radiation unit 2, a short-circuit pin 4, a coaxial cable 5, and a ground plate 9. Power is fed to the radiation unit 2 through the coaxial cable 5, and the radiation unit 2 is short-circuited to the ground plate 9 through the short-circuit pin 4, thereby achieving impedance matching. The PIFA must be designed in consideration of the length (L) of the radiation unit 2 and the height (H) of the antenna based on the width (Wp) of the short-circuit pin 4 and the width (W) of the radiation unit 2.
In this PIFA, among beams generated by the induced current to the radiation unit 2, beams directed toward a ground plane are re-induced, thereby reducing the beams directed toward the human body and improving the SAR characteristic. Further, the beams induced toward the radiation unit 2 are increased. This PIFA functions as a square-shaped micro-strip antenna with the length of the radiation unit 2 reduced to half, achieving a low profile structure. Further the PIFA is an internal antenna installed in the mobile communication terminal, thereby being aesthetically designed and protected from external impact.
In order to satisfy the trend of multi-functionality, the PIFA has been variously modified. Particularly, a dual band chip antenna, which is operable at different frequency bands, has been developed.
FIG. 2a is a schematic view of a conventional internal F-type dual band antenna.
With reference to FIG. 2a, the conventional F-type dual band chip antenna 10 comprises a radiation unit 20, a power feed pin 25, and a ground pin 26. The radiation unit 20 of the conventional F-type dual band chip antenna includes a high-band radiation unit 21 for processing a signal at a high band, which is located at the central area, and low-band radiation units 22, 23 and 24 for processing a signal at a low band, which are spaced from the high-band radiation unit 21 by a designated distance along the outer side of the high-band radiation unit 21. That is, the low-band radiation units 22, 23 and 24 are connected to the high-band radiation unit 21 in parallel. The power feed pin 25 and the ground pin 26 are connected to one end of the radiation unit 20.
FIG. 2b is a schematic view illustrating a current path in the conventional internal F-type dual band antenna.
As shown in FIG. 2b, currents 27 and 28 are respectively introduced into the high-band radiation unit 21 and the low-band radiation units 22, 23 and 24 through the power feed pin 25. The high-band radiation unit 21 radiates a radio wave of a high frequency signal by means of the current 27 introduced into the high-band radiation unit 21. Further, the low-band radiation units 22, 23 and 24 radiate radio waves of low frequency signals by means of the current 28 introduced into the low-band radiation units 22, 23 and 24.
The above conventional internal F-type dual band antenna is generally employed in a bar-type terminal having a large space for the antenna. However, the conventional F-type antenna has a large size, thus requiring a comparatively large storage space in the terminal. Further, in case that the conventional F-type antenna is manufactured in a small size, a usable frequency band of the antenna is narrowed and the antenna is negatively influenced by external stresses, i.e., the deterioration of the gain of the antenna. Particularly, in case that the above internal F-type dual band antenna is employed in a folder type terminal having a small size, the antenna is easily influenced by the human body, i.e., a position of a user's hand gripping the terminal. In this case, mute is generated during terminal communication, thereby preventing conversation via the terminal.